1. Field of the Invention
The present invention relates to a successive hole boring machine for boring successive holes (a slit-like hole formed by a plurality of successive holes) into a rock, and to a device for preventing a lost strike of the successive hole boring machine.
2. Prior Art
As machines for boring successive holes for the purposes of constructing an underground continuous wall, and digging an underground space such as an underground tank, an underground tunnel, and the like, there are known in the art to use a multiaxis auger (Japanese Patent Publication No. 59-18492), a multiplicity of rock drills installed on a slide frame in parallel to one another (Japanese Patent Laid-Open Publication No. 54-126601), rock drills displaced from each other respectively forward and backward to reduce a rod interval with respect to a width of the rock drill (Japanese Patent Laid-Open Publication No. 57-116893).
However, in the machine which uses the multiaxis auger, it is difficult to drill a hard rock layer. In the case where the multiplicity of rock drills are installed on a slide frame in parallel to one another, the mounting of the rock drills on the slide frame is troublesome, and furthermore, since the rod interval is larger than the width of the rock drill, bits of large diameter are needed to bore the successive holes and the loss of energy is large. The rod interval will be reduced to some extent when the rock drills are displaced from one another forward and backward, however, since the length of the slide frame between the front side and the rear side is increased, a large and long guide shell will be necessary. Furthermore, hoses for respectively supplying pressurized air or pressurized oil to the multiplicity of rock drills must be connected thereto. Thus, not only the piping work is troublesome but also the hoses disturb the boring work and the hoses are apt to be damaged.
On the other hand, in the prior art, where a breaker is used, at the time when the object to be broken is broken, a chisel slips out and a lost strike, or ineffective strike occurs resulting in the damage of a breaker main body. As a countermeasure, a lost strike preventing device (Japanese Utility Model Laid-Open Publication Nos. 51-39601 and 54-72781) is known in which based on the determination of the cause as being the thrust of the breaker applied to the object to be broken, a fluid passage for striking the breaker is automatically opened and closed. In another lost strike preventing device (Japanese Utility Model Laid-Open Publication No. 55-17791), when a striking piston advances beyond a normal striking point, a feed port of working fluid is interrupted to stop the driving of the piston.
In contrast, in the case of a rock drill, since a bit positioned at a front end of a rod always reaches the rock by applying thrust by a feed mechanism at the time of boring holes, there is no fear of lost striking and a lost strike preventing device is not particularly required.
However, in a successive hole boring machine in which a plurality of rods having bits attached to front ends thereof are disposed in parallel so that adjacent bits are somewhat displaced from one another forward and backward (or longitudinally, or axially), and each rod is connected to a shank rod which is inserted into a striking device, in performing a spot facing work or the like, the bits located at backward positions do not reach the rock in some cases. As a result, when the striking operation is performed in this condition, a lost strike will take place, and loosening of screws at connection portions of the rods, breakage of rods, damage of the striking device or bits will be caused.
Accordingly, the lost strike preventing device is required. However, in the successive hole boring machine, the striking device having a plurality of shank rods inserted therein is fixed to the single slide frame, and the thrust is applied by the feed mechanism. Thus, it is difficult to detect the thrust for each of the rods, and it is impossible to adopt the lost strike preventing device of the type in which by detecting the cause in the thrust and to automatically open and close the striking fluid passage. Furthermore, in the successive hole boring machine, in order to reduce the consumption of energy at the time of performing the boring work for successive holes, it is necessary to narrow the interval between the rods and to reduce the bit diameter. However, in the lost strike preventing device designed to stop the driving of the piston by interrupting the feed port of the working fluid when the striking piston advances beyond the normal striking point, there is a problem in which the structure of the striking device is complicated and large in size, and the interval between the rods cannot be narrowed.